Extrusion blow molding is a technique for making hollow thermoplastic shapes, and ultrasonic mechanical vibration or energy is used to fuse thermoplastics for cutting and jointing. The basic principles of each are described by the Modern Plastics Encyclopedia, 1979-1980, in its blow molding section, starting on page 230, and its ultrasonic assembly section, starting on page 432, this text by this reference to it being made a part of the present disclosure.
Briefly stated, extrusion blow-molding comprises extruding a thermally plasticized thermoplastic tubular parison on which a two-part blow molding die having an appropriately shaped cavity is closed so as to flatten the parison, pressurized air is injected into the parison portion encompassed by the cavity so as to blow mold this portion into the mold cavity, the parison quickly cools to at least a semi-rigid condition, and the mold is then opened and the blown parison removed. The mold parts are each formed with one-half of the molding cavity and they may have a protruding edge or pinch-off so that when the mold parts close on the parison, only a compressed thin film or flash connects the molded shape to the remaining portion of the parison, permitting easy subsequent trimming. Multi-cavity molds are used for small shapes.
For ultrasonic fusion an electrically powered ultrasonic mechanical vibrator connects with a horn which is resonant at the frequency used, absorption of this energy by a thermoplastic contacted by the horn resulting in heating causing fusion. This has been used for jointing thermoplastic parts. For an energy director joint, one of the parts is molded with a small triangular bead which directs the ultrasonic energy or vibration to the other of two parts being jointed together.
Some hollow shapes must be produced at low cost and in large quantities, as exemplified by Christmas tree balls. In this case a multi-cavity mold can be used. The pinch-off, if necessary, can be made so that a thicker flash results, or not used at all, so that when the die opens, the flattened parison and its multiplicity of blown balls can be handled without the balls falling and causing collection problems. The plastic is solidified because although not mentioned before, the dies are always cooled forcibly, as by water cooling, to reduce the die-closed interval to a minimum; but the parison and its retained balls are limber or floppy so as to make handling difficult. To correct this, molds have been designed so that the blow passages leading from the air supply to the parison portions in the cavities are enlarged so that a rigidifying spine is blow molded into the parison with its terminating ends opening into the blown shapes. These terminating ends interrupt the peripheral continuity of the flash holding the shapes to the blown parison.
In the above practice the flash holding the blown shape connected to the parison is called a gate and degating is required to free the shapes. Degating by die cutting results in large accumulations of plastic dust or fragments. After removal of the blown shapes, parisons are customarily reprocessed and returned to the parison extruder. This practice is made difficult in the case of dust or fragments. The use of a complete die-cut in the die is impractical because of the multiplicity of falling balls or other shapes involved. In addition, the die parts must be provided with die cutting tools with their consequent wear and sharpening problems, and there is the requirement that the die parts must exactly register parallel together during die closing, keeping in mind that commercially available blow-molding machines do not usually close a die precisely and with its two parts exactly parallel with each other as required for die cutting.
This need for exact registration also prevails when the shapes, such as the Christmas tree balls, are degated by using a die-cutting press. When applied to the making of Christmas tree balls, this has required an operator who manually effects the necessary precise registration of the blown parison relative to the die-cutting tools, eliminating the possibility for a completely automated production line.
A serious problem connected with mechanical or die cutting degating has been that it is impossible to produce shapes which do not show a fragmentation line where the die cut has been made. In the case of Christmas tree balls to be ultimately covered by decorative threads or yarn to give the familiar, normally red, apparently satin covered balls, fragmentation lines have been visibly apparent from outside of the covering. Such balls have been made of solid moldings of light-weight plastic material.
The object of the present invention is to enable the making of injection-blow molded hollow shapes, exemplified by Christmas tree balls, without involving the difficulties described above, and to provide a new form of extrusion blow-molded shape having an exterior which is free from evidence of die cutting, and in the case of Christmas tree balls, can be covered to provide flawless, satin-like covering without requiring the use of solid plastic balls. Ancillary to this, is the provision of the possibility for complete automation, including the degating step, by avoiding the need for exact or precise registration between the parison gates and the degating tools used. Furthermore, complete die cutting in the die with its attendant difficulties of handling the released shapes upon die opening, is to be avoided.